Resistance of Friction Connections with Open Slotted Holes in Towers for Wind Turbines

Sammanfattning: The most common solutions for in-situ connections of steel tubular towersegments of onshore wind turbines are ring flange connections. These havecertain drawbacks, such as e.g. costly production, long delivery time and ratherlow fatigue endurance. Friction connections with long open slotted holes havepreviously been proven to be a competitive alternative. In this work, the newtype of connection is investigated in various scales: segment tests, down-scaledexperiments and full-scale models. The influence of tower cross-section shape,execution tolerance (horizontal gap between the tower segments) and length ofthe connection on the bending resistance is thoroughly studied. In addition,buckling behaviour of the shell in the vicinity of the friction connection intowers with circular and polygonal cross-sections is analysed in order to checkpossible advantages of either cross-section. The influence of two types of theexecution tolerances on the connection strength is investigated: inwards bent“fingers”, leading to inclined gaps, and a parallel gap created by differentdiameters of the tower segments. Based on validated finite element analysesrecommendations for execution tolerances are proposed.A closer look is taken at the level of bolt forces under load application for thenew friction connections as well as for the ring flange connection. For theformer case, the influence of slotted holes on the joint resistance is checked.For both types of connections, comparison is drawn to hand calculation modelsused in engineering practice. Additionally, the distribution of meridionalstresses in the shell in the vicinity of the connection is studied.Based on the findings from the above described investigations,recommendations for the design of friction connections with open slotted holesin steel tubular towers for onshore wind turbines are given.